Biaxially oriented polypropylene film with cold sealing adhesive

ABSTRACT

The invention relates to a multilayered biaxially oriented polypropylene film with a resin-containing base layer and at least one first covering layer having a cold sealing adhesive coating on its outer surface.

The present invention relates to a polypropylene film having a coatingmade of cold sealing adhesives, which has very good sealing properties.

Biaxially oriented polypropylene films (boPP) are currently used aspackaging films in greatly varying applications. Polypropylene films aredistinguished by advantageous usage properties such as hightransparency, gloss, barriers to water vapor, good printability,rigidity, puncture resistance, etc. Sealing methods are used formanufacturing a package from these films, in which the outer layers ofthe films are stuck together using pressure and elevated temperature(hot sealing). For packaging temperature-sensitive products, coldsealing layers are used, which stick together at room temperature underonly pressure. Cold sealing adhesives of this type are based on naturalor synthetic latex compositions, which are applied to an outer filmsurface. The diametrically opposite side may not stick to or block withthe cold sealing adhesive during winding of the film thus coated and istherefore generally referred to as the release side. To ensure adequaterelease properties of this side, frequently, a suitable lacquer isapplied or it is laminated against a release film.

Adding additives to the film, which prevent sticking of the cold sealinglayer to the diametrically opposite surface, is also known. Thesemeasures have the disadvantage that the sealing properties of the coldsealing adhesive layer may be impaired.

Furthermore, it is known that a rough surface structure of the coldsealing side is advantageous for good anchoring of the cold sealinglayer on the film surface. Therefore, opaque films having avacuole-containing layer or having a pigmented layer are frequently usedfor these applications, which have greater surface roughness thantransparent films due to the vacuoles or the pigments. For someapplications, however, the high transparency of the polypropylene filmsis to be maintained. For these types, good anchoring of the cold sealinglayer is especially critical.

Various films have been suggested in the related art which may beadvantageously provided with a cold sealing adhesive coating. U.S. Pat.No. 5,482,780 describes a film having good adhesion in relation to coldsealing adhesives, which simultaneously has good release properties onthe diametrically opposite side. The film is multilayered and has twocover layers on the base layer, which is made of polypropylene. Therelease cover layer comprises a blend which contains anethylene-propylene copolymer which has a low ethylene content of 2 to8%. The second component of the blend is an ethylene-butylene copolymerwhich contains 0.5 to 6% ethylene. A non-migrating lubricant is added tothis side. The diametrically opposite cover layer, which is coated withthe cold sealing adhesive, is synthesized from an ethylene-propylenecopolymer which is corona treated to improve the cold sealing adhesiveadhesion. The adhesion of the cold sealing adhesive and the sealingproperties of the cold sealing adhesive are in need of improvement.

U.S. Pat. No. 6,022,612 also describes a multilayered biaxially orientedpolypropylene film having a cover layer for cold sealing coatings. Thiscover layer is made of a blend of polyolefin and a block copolymer.Block copolymers are, for example, styrene-isoprene-styrene blockcopolymers or styrene-butadiene-styrene block copolymers. This coverlayer has a matte appearance, but good adhesion of the cold sealingadhesive. Because of the cover layer mixture, the films have elevatedturbidity, which is not accepted for all applications.

U.S. Pat. No. 5,981,047 also describes a matte film for cold sealingadhesive applications. The film has a release cover layer, which issynthesized from a blend. An essential component of this blend is anethylene-butylene copolymer.

U.S. Pat. No. 6,074,731 describes a multilayered film having a coverlayer for cold sealing coatings, which comprises a blend of twocomponents. One component of the blend is an HDPE. The second blendcomponent is a polyisobutylene. These cover layers are to have goodadhesion in relation to cold sealing adhesives.

The object of the present invention is to provide a biaxially orientedpolypropylene film which is especially well suitable for cold sealingadhesive applications. The film is to have a surface which may beprovided with a cold sealing adhesive and has good adhesion in relationto the cold sealing adhesive. Furthermore, the cold sealing adhesive isto have good sealing strength in relation to itself.

Other film properties which are required in regard to the use as apackaging film may not be influenced disadvantageously. The film is tohave high gloss, no visual defects in the form of pinholes or bubbles,interference-free running on rapidly running packaging machines, and lowfilm turbidity. This object is achieved by a multilayered transparentbiaxially oriented polypropylene film made of a base layer and at leastone first cover layer, whose characterizing features are that the baselayer contains a hydrocarbon resin and the surface of the first coverlayer has a cold sealing adhesive coating.

The base layer of the film generally contains 80 to <100 weight-percent,particularly 85 to 95 weight-percent propylene polymer, in relation tothe base layer in each case.

In general, the propylene polymer contains at least 90 weight-percent,preferably 94 to 100 weight-percent, particularly 98 to 100weight-percent propylene. The corresponding comonomer content of at most10 weight-percent, or 0 to 6 weight-percent, or 0 to 2 weight-percent,respectively, generally comprises ethylene, if it is present. Thespecifications in weight-percent each relate to the propylene polymer.

Isotactic propylene homopolymers having a melting point of 140 to 170°C., preferably 155 to 165° C., and a melt-flow index (measurement DIN 53735 at 21.6 N load and 230° C.) of 1.0 to 10 g/10 minutes, preferably1.5 to 6.5 g/10 minutes, are preferred. The n-heptane-soluble componentof the polymers is generally 1-10 weight-percent, preferably 2-5weight-percent, in relation to the starting polymers. The molecularweight distribution of the propylene polymers may vary within widelimits. The ratio of the weight mean M_(w) to the numerical mean M_(n)is generally between 1 and 15.

It is essential to the present invention that the base layer contains ahydrocarbon resin (also referred to as a “hard resin” in English),preferably in a quantity of 5 to 20 weight-percent, particularly 8 to 15weight-percent, in relation to the weight of the base layer.

In principle, synthetic resins or resins of natural origin, which aregenerally partially or completely hydrogenated, come into considerationas hydrocarbon resins. The softening point of the resins is generallyabove 80° C. (measured according to DIN 1995-U4 and/or ASTM E-28), suchresins having a softening point of 100 to 180° C., particularly 110 to160° C., being preferred. In general, the hydrocarbon resins have a meanmolecular weight Mn between 500 and 2500 (Mw 500 to 3000) and thereforediffer from long-chain high molecular weight polymers, whose Mw (weightmean) is generally in the magnitude of 10,000 to multiples of 100,000.In addition, resins are amorphous substances which are glass-like andbrittle at room temperature, because of which they are also referred toas hard resins. Because of these characteristics, resins of polymers,particularly of propylene polymers, polyethylenes, and similar highmolecular weight substances, are different. For the purposes of thepresent invention, resins having a Mw of 600 to 1200 and a softeningpoint of 100 to 140° C. are particularly preferred.

As defined in the present invention, hydrocarbon resins include, forexample, petroleum resins, styrene resins, cyclopentadiene resins, andterpene resins (these resins are described in Ullmanns Encyklopädie dertechn. Chemie [Encyclopedia of Technical Chemistry], 4th edition, Volume12, pages 525 through 555).

The petroleum resins are those hydrocarbon resins which are manufacturedthrough polymerization of deep-decomposed petroleum materials in thepresence of a catalyst. These petroleum materials typically contain amixture of resin-producing substances such as styrene, methylstyrene,vinyl toluene, indene, methyl indene, butadiene, isoprene, piperylene,and pentylene. The styrene resins are homopolymers of styrene orcopolymers of styrene with other monomers such as methylstyrene, vinyltoluene, and butadiene. The cyclopentadiene resins are cyclopentadienehomopolymers or cyclopentadiene copolymers, which are obtained from coaltar distillates and decomposed petroleum gas. These resins aremanufactured by keeping the materials which contain cyclopentadiene athigh temperature for a long time. Dimers, trimers, or oligomers may beobtained as a function of the reaction temperature.

The terpene resins are polymers of terpenes, i.e., hydrocarbons of theformula C₁₀H₁₆, which are contained in almost all ethereal oils or oilyresins of plants, and phenol-modified terpene resins. Pinene, α-pinene,dipentene, limonene, myrcene, camphene, and similar terpenes are to becited as special examples of the terpenes. The hydrocarbon resins mayalso be modified hydrocarbon resins. The modification is generallyperformed through reaction of the raw materials before polymerization,by introducing special monomers, or through reaction of the polymerizedproduct, hydrogenation or partial hydrogenation particularly beingperformed.

In addition, styrene homopolymers, styrene copolymers, cyclopentadienehomopolymers, cyclopentadiene copolymers, and/or terpene polymers havinga softening point of above 135° C. in each case are additionally used asthe hydrocarbon resins (for the unsaturated polymers, the hydrogenatedproduct is preferred). The cyclopentadiene polymers having a softeningpoint of at least 140° C. or copolymers made of α-methylstyrene andvinyl toluene having a softening point of 120 to 150° C. are veryespecially preferably used in the base layer.

In addition to the component of resin essential to the presentinvention, the base layer may contain typical additives, preferablyneutralization agents and stabilizers, as well as possibly additionalantistatic agents and/or lubricants in the particular effectivequantities, no vacuole-initiating fillers or pigments being added to thebase layer, however, since the base layer is to be transparent.

Preferred antistatic agents are alkali-alkanesulfonates and/or theessentially straight-chain and saturated aliphatic, tertiary amineshaving an aliphatic residue having 10 to 20 carbon atoms, which aresubstituted by ω-hydroxy-(C₁-C₄)-alkyl groups,N,N-bis-(2-hydroxyethyl)-alkyl amines having 10 to 20 carbon atoms,preferably 12 to 18 carbon atoms, in the alkyl residue being especiallysuitable. The effective quantity of antistatic agent is in the rangefrom 0.05 to 0.5 weight-percent. Furthermore, glycerin monostearate maybe used as an antistatic agent in a quantity of 0.03% to 0.5%.

Lubricants are higher aliphatic acid amides, higher aliphatic acidesters, waxes, and metal soaps. The effective quantity of lubricant isin the range from 0.01 to 3 weight-percent, preferably 0.02 to 1weight-percent. The addition of higher aliphatic acid amides in therange from 0.01 to 0.25 weight-percent in the base layer is especiallysuitable. A particularly suitable aliphatic acid amide is erucic acidamide.

The typical stabilizing compounds for ethylene, propylene, and otherα-olefin polymers may be used as stabilizers. The quantity added isbetween 0.05 and 2 weight-percent. Phenolic stabilizers,alkaline/alkaline earth stearates, and/or alkali/alkaline earthcarbonates are especially suitable. Phenolic stabilizers are preferredin a quantity from 0.1 to 0.6 weight-percent, particularly 0.15 to 0.3weight-percent, and having a molar mass of more than 500 g/mole.Pentaerythrityl-tetrakis-3-(3,5-di-tertiarybutyl-4-hydroxy-phenyl)-propionate or1,3,5-trimethyl-2,4,6-tris(3,5-di-tertiary butyl-4-hydroxy benzyl)benzol are especially advantageous.

Neutralization agents are preferably dihydrotalcite, calcium stearate,and/or calcium carbonate of a mean particle size of at most 0.7 μm, anabsolute particle size of less than 10 μm, and a specific surface areaof at least 40 m²/g.

The above specifications in percent each relate to the weight of thebase layer.

The polypropylene film according to the present invention comprises atleast one cover layer, generally made of propylene polymers. Propylenehomopolymers or propylene mixed polymers which predominantly containpropylene are suitable. In general, the cover layer contains at least 80weight-percent, preferably 85 to 100 weight-percent, particularly 95 to<100 weight-percent propylene polymers, in relation to the weight of thecover layer.

Isotactic propylene homopolymers generally have a melting point of 140to 170° C., preferably 155 to 165° C., and a melt-flow index(measurement DIN 53 735 at 21.6 N load and 230° C.) of 1.0 to 10 g/10minutes, preferably from 1.5 to 6.5 g/10 minutes. The n-heptane-solublecomponent of the propylene homopolymers is generally 1 to 10weight-percent, preferably 2-5 weight-percent, in relation to thestarting polymer.

Propylene mixed polymers predominantly contain propylene units,preferably in a quantity of at least 80 weight-percent, in relation tothe propylene copolymer or propylene terpolymer. Butylene or ethyleneare preferred as the comonomers, whose proportion is accordingly up to20 weight-percent. Examples of preferred propylene copolymers orpropylene terpolymers are random ethylene-propylene copolymers having anethylene content of 1 to 10 weight-percent, preferably 2.5 to 8weight-percent, or random propylene-butylene-1 copolymers having abutylene content of 2 to 25 weight-percent, preferably 4 to 20weight-percent, each in relation to the total weight of the copolymer,or random ethylene-propylene-butylene-1 terpolymers having an ethylenecontent of 1 to 10 weight-percent, preferably 2 to 6 weight-percent, anda butylene-1 content of 2 to 20 weight-percent, preferably 4 to 20weight-percent, each in relation to the total weight of the terpolymer,or a blend or a mixture made of the cited propylene copolymers orpropylene terpolymers, a blend made of an ethylene-propylene-butylene-1terpolymer and a propylene-butylene-1 copolymer having an ethylenecontent of 0.1 to 7 weight-percent and a propylene content of 50 to 90weight-percent and a butylene-1 content of 10 to 40 weight-percent, eachin relation to the total weight of the polymer blend, being especiallypreferred.

The copolymers and/or terpolymers described above, which are used in thecover layer, generally have a melt-flow index of 1.5 to 30 g/10 minutes,preferably from 3 to 15 g/10 minutes. The melting point is in the rangefrom 120 to 140° C. The blend of copolymers and terpolymers describedabove has a melt-flow index of 5 to 9 g/10 minutes and a melting pointof 120 to 150° C. All melt-flow indices specified above were measured at230° C. and a force of 21.6 N (DIN 53 735).

If necessary, the additives described above for the base layer may beadded to the cover layer, of which antistatic agents, neutralizationagents, and/or stabilizers, as well as antiblocking agents, arepreferred. The specifications in weight-percent then relatecorrespondingly to the weight of the cover layer.

Suitable antiblocking agents are inorganic accessory agents such assilicon dioxide, calcium carbonate, magnesium silicate, aluminumsilicate, calcium phosphate, and the like and/or incompatible organicpolymers such as polyamides, polyesters, polycarbonates, and the like,benzoguanamine-formaldehyde polymers, silicon dioxide, and calciumcarbonate being preferred. The effective quantity of antiblocking agentis in the range from 0.1 to 2 weight-percent, preferably 0.1 to 0.8weight-percent. The mean particle size is between 1 and 6 μm,particularly 2 and 5 μm, particles having a spherical shape, asdescribed in EP-A-0 236 945 and DE-A-38 01 535, being especiallysuitable.

It has been found that a resin-modified film may be used especiallyadvantageously for coating with cold sealing adhesive. The adhesion ofthe cold sealing adhesive to the surface of the cover layer issignificantly improved by the modification of the base layer using ahydrocarbon resin. The seal seam strength of the cold sealing adhesiveis also significantly greater than in films without hydrocarbon resinadded. Although the use of resins for modifying film properties is knownper se, it was surprising that resin-modified films of this type havebetter cold sealing adhesive adhesion and sealing.

The film according to the present invention comprises at least the baselayer described above and a first cover layer, which is provided withcold sealing adhesive in the use according to the present invention. Ingeneral, the diametrically opposite surface of the film must have arelease effect in relation to the cold sealing adhesive, so that thecold sealing adhesive does not stick to this second surface duringwinding of the film into a roll, for example. This release may beimplemented by a second diametrically opposite cover layer, for example,in which the release in relation to the cold sealing adhesive is ensuredthrough a suitable formulation, for example, by adding wax. However, itis also possible to provide the surface of the diametrically oppositesecond cover layer or the surface of the base layer with a releaselacquer or to laminate this surface against a further release film.Suitable release layers or technologies are known in the related art.

In a preferred embodiment, a second cover layer is applied to thediametrically opposite surface of the base layer, the construction,thickness, and composition of a second cover layer being selectedindependently from the first cover layer already provided. The secondcover layer may be synthesized in principle from the same propylenepolymers described for the first cover layer, or if necessary otherpolyolefinic polymers and polymer mixtures may also be selected.

The thickness of the first cover layer(s) is generally greater than 0.1μm and is preferably in the range from 0.3 to 3 μm, particularly 0.4 to1.5 μm. The thickness of the second cover layer is in a similar order ofmagnitude, preferably 0.5 to 2 μm. The total thickness of thepolypropylene film according to the present invention may vary withinwide limits and depends on the intended use. It is preferably 4 to 60μm, particularly 5 to 30 μm, preferably 6 to 25 μm, the base layercorresponding to the difference between the total film thickness and thethickness of the cover layers.

According to the present invention, the film is provided with coldsealing adhesive on the surface of the first cover layer. Cold sealingadhesive layers differ in principle from hot sealing layers in that thesealing is performed at room temperature only through the application ofpressure. The packaged product therefore does not experience anytemperature stress. The cold sealing adhesive does not requireactivation by water, solvent, or heat. Cold sealing adhesives are knownper se in the related art and are based, for example, on synthetic orartificial latex or rubber, polyurethane, or acrylic polymers. Thesesubstances are applied to the film surface from suitable, possiblyaqueous solutions. All typical application methods for cold sealingadhesives are suitable, such as gravure printing. Numerous patentspecifications describe cold sealing adhesives and improved compositionsand the corresponding application methods; for example, in U.S. Pat. No.3,740,366, U.S. Pat. No. 3,299,010, U.S. Pat. No. 4,012,560, U.S. Pat.No. 4,387,172, U.S. Pat. No. 4,889,884, U.S. Pat. Nos. 4,902,370,4,810,745, U.S. Pat. No. 4,902,370, or U.S. Pat. No. 4,851,459.Depending on the intended application, the cold sealing adhesive may beapplied in sections or over the entire area. If necessary, the surfaceof the cover layer to be coated may be pretreated using corona, flame,or plasma to improve the cold sealing adhesive adhesion before theapplication of the adhesive.

Furthermore, the present invention relates to a method for manufacturingthe polypropylene film according to the present invention according tocoextrusion methods known per se.

In the framework of this method, the melts corresponding to the layersof the film are coextruded through a sheet die, the film thus obtainedis drawn off to solidify on one or more roll(s), the film issubsequently biaxially stretched (oriented), the biaxially stretchedfilm is thermally fixed and possibly corona or flame treated on thesurface layer provided for treatment.

Biaxial stretching (orientation) is performed sequentially, sequentialbiaxial stretching, in which stretching is first performedlongitudinally (in the machine direction) and then transversely(perpendicular to the machine direction), being preferred.

Firstly, as is typical in coextrusion methods, the polymer and/or thepolymer mixture of the individual layers is compressed and liquefied inan extruder, the possibly added additives already able to be containedin the polymer and/or in the polymer mixture. The melts are thenextruded simultaneously through a sheet die, and the extrudedmultilayered film is drawn off on one or more draw-off rolls, so that itcools and solidifies.

The film thus obtained is then stretched longitudinally and transverselyto the extrusion direction, which results in orientation of themolecular chains. The longitudinal stretching is expediently performedwith the aid of two rolls running at different speeds corresponding tothe desired stretching ratio and the transverse stretching is performedwith the aid of a corresponding tentering frame. The longitudinalstretching ratios are in the range from 4 to 8, preferably 5 to 6. Thetransverse stretching ratios are in the range from 5 to 10, preferably 7to 9.

The biaxial stretching of the film is followed by its thermal fixing(heat treatment), the film being held approximately 0.1 to 10 secondslong at a temperature of 100 to 160° C. The film is subsequently woundup in a typical way using a winding device.

It has been shown to be especially favorable to keep the draw-off rollor rolls, through which the extruded film is cooled and solidified, at atemperature from 10 to 100° C., preferably 20 to 50° C., through aheating and cooling loop.

The temperatures at which longitudinal and transverse stretching isperformed may vary in a relatively broad range and depend on the desiredproperties of the film. In general, the longitudinal stretching ispreferably performed at 80 to 150° C. and the transverse stretching ispreferably performed at 120 to 170° C.

One or both surfaces of the film is/are preferably corona or flametreated according to one of the known methods after the biaxialstretching. The treatment intensity is generally in the range from 37 to50 mN/m, preferably 39 to 45 mN/m.

In the corona treatment, the procedure is expediently that the film isguided between two conductor elements used as electrodes, such a highvoltage, usually AC voltage (approximately 5 to 20 kV and 5 to 30 kHz)being applied between electrodes that spray or corona discharges mayoccur. Through the spray or corona discharge, the air above the filmsurface is ionized and reacts with the molecules of the film surface, sothat polar intercalations arise in the essentially non-polar polymermatrix.

For a flame treatment using polarized flame (compare U.S. Pat. No.4,622,237), an electrical DC voltage is applied between a burner(negative pole) and a cooling roll. The level of the applied voltage isbetween 400 and 3000 V, and is preferably in the range from 500 to 2000V. The ionized atoms receive elevated acceleration due to the appliedvoltage and hit the polymer surface with greater kinetic energy. Thechemical bonds within the polymer molecules are broken more easily, andthe radical formation occurs more rapidly. The thermal strain of thepolymers is much less in this case than in standard flame treatment, andfilms may be obtained in which the sealing properties of the treatedside are even better than those of the untreated side.

The following measurement methods were used to characterize the rawmaterials and the films:

Melt-Flow Index

The melt-flow index was measured according to DIN 53735 at 21.6 N loadand 230° C.

Melting Point

DSC measurement, maximum of the melting curve, heating speed 20°C./minute.

Turbidity

The turbidity of the film was measured according to ASTM-D 1003-52.

Gloss

The gloss was determined according to DIN 67 530. The reflector valuewas measured as the optical characteristic for the surface of the film.The angle of incidence was set at 60° or 85° in accordance with thestandards ASTM-D 523-78 and ISO 2813. A light beam is incident on theflat testing surface at the set angle of incidence and is reflectedand/or scattered by the surface. The light beams incident on thephotoelectronic receiver are displayed as a proportional electricalvariable. The measured value is dimensionless and must be specified withthe angle of incidence.

Surface Tension

The surface tension was determined via the ink method according to DIN53364.

Friction

The friction was measured according to DIN 53375.

Viscosity

The viscosity was determined according to DIN 53018, and/or according toDIN 53019.

Cold Sealing Seam Strength

To determine the cold sealing seam strength, two surfaces coated withcold sealing adhesive were “sealed” with one another. For this purpose,two 15 mm wide film strips were laid one on top of another and pressedin a sealing device HSG from Brugger at room temperature and a sealingtime of 0.5 seconds and a sealing pressure of 30 N/cm². Subsequently,the two strips were pulled apart according to the T-peel method. In thiscase, the force-path diagram during peeling was measured in a typicalway. The maximum force before the tearing of the sealed sample wasspecified as the initial tearing strength.

Cold Sealing Layer Anchoring

The anchoring of the cold sealing layer on the film surface wasinvestigated using an adhesive strip test. For this purpose, an adhesivetape (e.g., from Scotch) was pressed onto the cold sealing layer.Subsequently, in a tensile testing machine, the adhesive tape was pulledoff of the film coated with cold sealing adhesive again, the force-pathdiagram during pulling off being measured in a typical way. In the eventof complete transfer of the cold sealing adhesive layer to the adhesivetape, the force which is necessary to pull off the adhesive tape andcold sealing layer corresponds to the anchoring of the cold sealinglayer.

The present invention will now be explained on the basis of examples:

EXAMPLE 1

A transparent three-layered film having cover layers on both sideshaving a total thickness of 20 μm was manufactured through coextrusionand subsequent step-by-step orientation in the longitudinal andtransverse directions. The cover layers had a thickness of 0.6 μm each.

Base Layer

87.79 weight-percent isotactic propylene homopolymer having a meltingpoint of 163° C. and a melt-flow index of 3.4 g/10 minutes.

12.0 weight-percent hydrocarbon resin having a softening point of 120°C. and a mean molecular weight Mw of 1000

0.15 weight-percent N, N-bis-ethoxy alkylamine (antistatic agent)

0.06 weight-percent erucic acid amide

Cover Layer 1:

approximately

99.67 weight-percent random ethylene-propylene-butylene terpolymerhaving an ethylene content of 3 weight-percent and a butylene content of7 weight-percent (remainder propylene)

0.33 weight-percent SiO₂ as an antiblocking agent having a mean particlesize of 2 μm

Cover Layer 2:

approximately

99.37 weight-percent random ethylene-propylene copolymer having anethylene content of approximately 4.5 weight-percent and a softeningpoint of approximately 130° C.

0.33 weight-percent SiO₂ as an antiblocking agent having a mean particlesize of 2 μm

0.30 weight-percent polyethylene wax having a mean molecular weight Mnof 1000

The manufacturing conditions in the individual method steps were:

Extrusion: temperatures base layer: 260° C.

-   -   cover layers: 240° C.    -   temperature of the draw-off roll: 20° C.

Longitudinal stretching: temperature: 110° C.

-   -   longitudinal stretching ratio: 5.5

Transverse stretching: temperature: 160° C.

-   -   transverse stretching ratio: 9

Fixing: temperature: 140° C.

-   -   convergence: 20%

The transverse stretching ratio λ_(Q)=9 is an effective value. Thiseffective value is calculated from the final film width B, reduced bytwice the border strip width b, divided by the width of thelongitudinally-stretched film C, also reduced by twice the border stripwidth b.

EXAMPLE 2

A film was manufactured as described in Example 1. In contrast toExample 1, the resin content in the base layer was reduced from 12 to 8weight-percent. The remaining composition and the manufacturingconditions were not changed in relation to Example 1.

Comparative Example 1

A film was manufactured as described in Example 1. In contrast toExample 1, the film now contained no hydrocarbon resin in the baselayer. The remaining composition and the manufacturing conditions werenot changed in relation to Example 1.

All films according to the examples and the comparative example werecoated in the gravure printing method on the surface of cover layer 1with a cold sealing adhesive C2881 from Bostik Findley. The propertiesof the films according to the examples and the comparative example aresummarized in the following table: TABLE Resin Anchoring of content inthe cold base layer sealing Sealing weight- adhesive strength Examplepercent N/15 mm N/15 mm B1 12 4.5 4.3 B2 8 4.3 4.3 VB1 0 3.5 3.0B = example;VB = comparative example

1. A multilayered transparent biaxially oriented polypropylene film madeof a base layer and at least one first cover layer, characterized inthat the base layer has a hydrocarbon resin and the cover layer has acold sealing adhesive coating on its outer surface.
 2. The polypropylenefilm according to claim 1, characterized in that the base layer containsan isotactic polypropylene having a melting point of 155-165° C.
 3. Thepolypropylene film according to claim 1, characterized in that the baselayer contains the hydrocarbon resin in a quantity of 5 to 20weight-percent, in relation to the weight of the base layer.
 4. Thepolypropylene film according to claim 1, characterized in that thehydrocarbon resin contains a non-hydrogenated styrene polymer, amethylstyrene-styrene copolymer, a pentadiene polymer, a pentadiene andcyclopentadiene copolymer, cyclopentadiene polymer, an α-pinene polymer,β-pinene polymer, colophony or colophony derivatives or terpene polymersand hydrogenated compounds thereof, or hydrated α-methylstyrene-vinyltoluene copolymer or mixtures thereof.
 5. The polypropylene filmaccording to claim 1, characterized in that the hydrocarbon resin has asoftening point of 100 to 160° C.
 6. The polypropylene film according toclaim 1, characterized in that the first cover layer is synthesized fromisotactic propylene homopolymers, propylene copolymers, or propyleneterpolymers or mixtures of these polymers, the propylene copolymers andterpolymers having a propylene content of at least 80 weight-percent inrelation to the polymer.
 7. The polypropylene film according to claim 1,characterized in that the surface of the first cover layer is pretreatedusing corona, plasma, or flame.
 8. The polypropylene film according toclaim 1, characterized in that a second cover layer made of polyolefinicpolymers is applied to the diametrically opposite surface of the baselayer.
 9. The polypropylene film according claim 1, characterized inthat a release layer is applied to the surface diametrically oppositethe first cover layer as the outer layer, whose surface has a lowadhesion in relation to cold sealing coatings.
 10. The polypropylenefilm according to claim 1, characterized in that the release layer is arelease lacquer, a release film, or a second coextruded cover layer. 11.The polypropylene film according to claim 1, characterized in that thebase layer contains an antistatic agent.
 12. The polypropylene filmaccording to claim 1, characterized in that all layers of the filmcontain neutralization agents and stabilizers.
 13. The polypropylenefilm according to claim 1, characterized in that the first cover layercontains antiblocking agent.
 14. A method for manufacturing apolypropylene film according to claim 1, characterized in that thecoating of the biaxially oriented film with the cold sealing adhesive isperformed in the gravure printing method.
 15. The polypropylene filmaccording to claim 11, wherein said antistatic agent is tertiaryaliphatic amine.